1. Field of the Invention
This invention relates to electrodeposition coating systems, and more particularly to an electrodeposition coating system using a plurality of membrane electrodes.
2. Description of the Prior Art
In the electrodeposition coating, uniformity and adhesion of the coating film on an article are excellent, automatization and labor-saving can be effected easily and the degree of causing pollution is low, whereby, there have been widely applied to the coating film treatment of motor vehicle bodies and so forth for example, particularly as suitable ones in the prime coating or one coat finishing for the coating of metal materials. Coating materials used in the electrodeposition coating are broadly divided into two including an anion type coating material having an acid radical as a resin of basic composition (carboxyl group for example) and a cation type coating material having a base as a resin of basic composition (amine group for example). Either one, solely used, is very low in solubility in the water. Because of this, there have been used such ones that, in the case of the anion type coating material, an alkaline neutralizing agent such for example as triethylamine is mixed thereinto, while, in the case of the cation type coating material, an acidic neutralizing agent such as acetic acid is mixed thereinto, whereby the both coating materials are neutralized to form salts, so that the solubility in the water is raised.
The neutralizing agents are mixed in accordance with the properties of the resin components of the respective coating materials as described above. However, as the electrodeposition treatment of the article to be coated advances, the resin component in an aqueous solution decreases, whereby the coating material should be supplied from the outside. In this case, in the aforesaid aqueous solution, there is accumulated amine or acetic acid as the neutralizing agent and pH is gradually changed, and a phenomenon such as redissolving of the coated surface or occurrence of pin holes is generated. Because of this, recently, a so-called pH control is carried out to achieve the actual effect such that the electrodes on one side are separated from the article to be coated as being the other electrode by a membrane, and amine or acetic acid is electrically and osmotically extracted from the aqueous solution, to thereby prevent the neutralizing agent from increasing in the aqueous solution.
On the other hand, in order to raise the durability of the above-described membrane, the inventor of the present invention proposed tubular membrane electrodes as being a new system, wherein the membrane are formed into tubular shapes for use, and actually installed and was working it as a standing type.
However, in the electrodeposition coating, the outer surface of the article to be coated as a whole need not necessarily be uniformly coated. For example, such a disadvantage is presented that the thickness of the coating film may be thin on the inner side, the top surface or the under surface to a considerable extent. FIGS. 16(1) and 16(2) show the specific examples thereof. These FIGS. 16(1) and 16(2) show comparison of attached quantities of the coating film in the case where the electrodeposition coating is carried out on the motor vehicle body by the conventional technique. In FIGS. 16(1) and 16(2), indicated by (1), (2) and (3) are directions, respectively. In these FIGS. 16(1) and 16(2), in the directions (1), the thickness of the coating film on one and the same line, are generally uniform. In contrast thereto, in the directions (2) and (3), such a disadvantage is presented that, the more distant the coating film becomes, the thinner the coating film becomes sharply.
Restating the above, when coating a large article, such as a motor vehicle body, in an electrodeposition system, distance of the article to be coated from membrane electrodes tends to vary in the vertical direction (the direction indicated by double-headed arrows 2 in FIG. 16 (1)) rather than in the horizontal direction (the direction indicated by double headed arrows 1 in FIG. 16 (1)). Thus, systems which do not allow for control of voltage applied to membrane electrodes in the vertical direction cannot effectively uniformly coat such articles. For example, it is known to arrange membrane electrodes vertically. Such an arrangement is shown in FIG. 2 of DEOS No. 2,531,902. Further, it is known to apply different voltages to various groups of membrane electrodes. However, it is simply not possible with such arrangements to create a variable potential field in the vertical direction. It is only possible with such arrangements to create variable potential fields in the horizontal direction, in which direction, as stated above, it is generally not necessary to vary potential field strength.
And, in this case, if the electrodeposition coating is carried as referenced from a portion where the coating film is thin, such a disadvantage is presented that the thicknesses of the coating film on the both side surfaces opposed to the tubular membrane electrodes as being the electrodes on the other side become considerably large, so that disadvantageously increased consumption of the coating material leads to uneconomics, thus increasing the manufacturing cost.